Tandy (Radio Shack) Computers

Model 4

Overview

The TRS-80 Model 4, the sucessor to the Model III, became available in 1983, with a number of configurations to choose from:

No diskdrive and 16KB RAM $999

Single one-sided 180KB disk drive and 64KB RAM $1699

Two disk drives and 64KB RAM $1999

While the appearance of the Model 4 is close to that of the Model III, the Model 4 is larger, measuring 33.02 x 45.72 x 53.34 (13 X 18 1/2 X 21 in.), having a bigger keyboard, and the Model 4 is beige, unlike the Model III, which is black and grey. The Model 4 is powered by a 4MHz Z80, along with 16KB/64KB RAM as stock but upgradeable to 128KB, and is capable of displaying 80 X 24 characters. This is contrasted with the Model III's standard 2MHz Z80, maximum 48KB RAM, and 64 x 16 character display..

The TRS-80 Model 4 can be caught sight of in a number of shows and films from mid-80s to much more recent times, as highlighted on this site:

http://www.starringthecomputer.com/computer.html?c=70

For an unboxing, teardown and troubleshooting of a model 4, please have a look at the following two videos from Adrian's Digital Basement:

Resources

An Introduction to your Disk System (US version):

http://dunfield.classiccmp.org/trs80/m4intro.pdf

TRS 80 Model 4 Disk System Owner's Manual:

https://archive.org/details/TRS-80_Model_4_Disk_System_Owners_Manual/mode/2up

TRS-80 Model 4/4P Technical Reference Manual:

http://dunfield.classiccmp.org/trs80/m4tech.pdf

Technical

Overview of the different versions of the model 4:

https://www.trs-80.com/wordpress/trs-80-computer-line/model-4/

Toward the bottom of the page are fascinating insights in the development of the various model 4 variations.

Model 4P

Overview

The TRS-80 Model 4P (referred to by some as the Model 5 or Model 4C before release) is essentially a portable version of the Model 4 with very close compatibility, released on 15th November 1983, at $1799 ($200 less than the Model 4). There are two catalogue numbers for the machine, 26-1080/26-1080A, the difference between being that 26-1080 is the earlier, 'non-gate array' version, and 26-1080A is the later, gate array version. The Model 4P uses a Z80A CPU running at 4MHz, with 64KB main RAM (upgradeable to 128KB), as well as a 4KB boot ROM (there is no built-in BASIC), and buzzer for simple sound reproduction.

Coloured cream, and weighing at almost 12KG, the Model 4P measures (HWD) 23.6 x 41.9 x 33.7 cm, and features a 9 inch monochrome CRT (black and white for cat. 26-1080, green for cat. 26-1080A), smaller than the Model 4's 12 inch screen, however, the phosphor dots on the Model 4P's screen are closer together than on the Model 4's CRT, giving a very crisp image, and with the same pixel resolution of 640 x 240. The Model 4P also has two built-in 5.25" floppy disk drives - no more than two internal drives are officially supported - with drive 0 (the boot drive) closest to the CRT and drive 1 the furthest. There is a detachable keyboard that can be stored below the screen along with its cable, and a handle for carrying the computer about. As with many retro computers, the Model 4P is also prone to 'yellowing' over the years but can be restored to its former colour with retrobrighting or similar techniques.

The Model 4P is not totally compatible with the Model 4 as it lacks the Model III ROM, thus software designed for the Model III won't run without first loading a Model III ROM image from disk. Unlike the Model 4, the Model 4P doesn't have a cassette connection for an external tape recorder for file storage, however, the Model 4P routes what would be the cassette output to the built-in speaker so that Model III software that played sound through the cassette port still works.

As a luggable computer, the Model 4P (the 'P' in Model 4P likely meaning portable) can be carried around but when it needs to be used there are is a latch both sides (some Model 4Ps have metal latch, other plastic) which push outward, releasing the front cover part of the case, which has provision for holding the power cord, manual and disks. Looking at the now exposed computer you will find the brightness knob, contrast knob, power switch and reset switch at the left side, the CRT screen in the middle and two 5.25" floppy disk drives toward the right.

The keyboard, which pulls out from the bottom compartment, is labelled as 'Radio Shack TRS-80 MODEL 4P PORTABLE COMPUTER', and has supports that rotate outward to angle the keyboard. There are a total of 70 keys on the keyboard, which includes arrow keys arranged in the 'modern' pattern, and a numeric keypad with three function keys.

There are air vents on the top and front of the computer (visible with the front cover removed), you can feel cool air come out from top air vents when the computer is on. While powered on the computer is quite quiet, certainly not loud enough to be too distracting.

At the rear of the computer is the handle; as instructed, press on either end to release it so it can be grasped. Below the handle is a compartment door, push on the two tabs to release, revealing various connectors. In the next photo you can see the port cover lifted up, exposing a label with various information including the model number, serial number, and power requirements (note that this is the UK model):

The model number (26-1080A) - which is on a sticker that is peeling off - conveniently exposes that it is covering up the original (non-gate array) model number (26-1080). The ribbon cable you can see toward the bottom left of the above image appears to be an external floppy drive cable fitted by a previous owner.

There is a RS-232C connector (DE25 female), ribbon cable with IDC male 17x2 male connector on the end (possibly for an external floppy drive, the cable isn't standard), I/O bus (edge connector, 25 x 2) blanked off, power in (3 pin IEC) and parallel printer (edge connector 17x2). There is also large blanking plate above the ribbon cable, removing it reveals a recessed connector (edge connector, female 6x2) - the area where the connector is located and the connector itself is for an expansion card, such as a modem.

Peripherals for the Model 4P include a 300-baud modem (which fits inside the computer in a dedicated expansion area) and connects to the RS-232C port, printer (by way of the parallel printer connector), and an external hard drive (connects to the I/O bus).

When you power on the Model 4P it will try to boot off a disk in the first drive, if there is no disk present or the disk isn't suitable, you will get this message or something similar in English, German and French:

If you cannot load a known good disk suitable for the Model 4P (Model 4 discs should also boot) then the drive might be faulty, see the Repair and Troubleshooting sections for help.

Repair

The Model 4P contains high voltage circuits that can cause harm or death, only attempt repairs if you have experience with high voltage circuits and know how to protect yourself from danger.

General

Please see this video series for teardown, diagnostics and repair of a Model 4P (Adrian's Digital Basement):

Floppy disk

I bought my Model 4P from eBay as non-functional and while the computer powered on no problem, it wouldn't boot from a Model 4 DOS disk, the screen showing 'The Floppy Disk Drive is Not Available' error message. Taking the computer apart I found that the disk drives appear to have been installed at a later date because of the extra screw holes made in the metal holding them in place (it was common to upgrade the drives to dual sided). Drive 0 has the model number 'NF 5300' and drive 1 'SM596D', which produced no results online and there isn't even any branding on the drives as far as I can see. However, I found that drive 1 does work, by configuring it as drive 0 using the appropriate jumper on the drive, since the Model 4P will only boot off drive 0. Now I was able to successfully boot off my Model 4 DOS disk.

Having removed the bad drive from the computer and having done the usual maintenance - cleaning the heads, etc. - with no success I did much investigating, reverse engineering some of the circuit which contains unknown ICs, replacing the capacitors and transistor, but I came to the conclusion that the spindle motor (which is the direct drive type) is faulty. These particular drives spin up for about 10 seconds when power is applied to them or a disk is inserted, and these details I was able to confirm with the working drive, as not all 5.25" drives behave in that way. The faulty drive's spindle motor rarely spun up at power on but if I briefly spun it myself it would run as normal.

Going into more detail, the spindle motor board has just three inputs, +12V, 0V, and S (enable), and using my bench power supply I could power just the spindle motor board on its own, checking against the working drive to confirm how it was supposed to work. I could see that upon power up when the motor failed to start, about 700mA was being drawn on the 12V line instead of around 90mA for the working drive with the motor spinning. Additionally, the timing chip would stop producing signals at its outputs, possible because the motor drive IC was telling it there was a fault; the motor contains a number of sensors and the motor drive chip can probably detect excessive current draw. 

Another clue that the spindle motor was at fault was the fact that spinning it by hand (with no power connected) seemed to have more resistance and the sound was different to the spindle motor in the working drive. Finding a replacement spindle motor would be very difficult and unfortunately I was unable to get the motor apart due to a seized screw, so I had to accept there wasn't much I could do and look for a different drive.

I had a Torch Z80 Disc pack, which is a dual 5.25" disk drive unit (containing x2 Mitsubishi M4853 96 TPI/80 track drives) which connects to a BBC Micro along with an internally mounted Z80 second processor but since I typically use a GoTek with the BBC, I was willing to put the two drives into the Model 4P. Rather than just install the drives into the Model 4P and screw the machine back together, I wanted to make sure the drives worked by trying to boot off my Model 4 TRSDOS 06.01.02 disk. It was just as well I checked as the computer refused to boot off the disk. So I connected the working drive that came with the Model 4P as drive 0 and one of the Torch drives as the second drive and was able to boot, and I found that I could format a disk in the second drive as a high capacity disk. Looking online, and perhaps I'd forgotten this, I discovered that you can't read 48 TPI formatted disks in a 96 TPI drive, at least not without double stepping, and I had no idea how to get the Model 4P to do that. The solution was to create a boot disk in 96 TPI, double sided format from the 48 TPI disk, and I came across this site:

https://www.trs-80.com/wordpress/tips/dos/

In the section 'Formatting A Double-Sided Diskette Under TRSDOS 6.x / LDOS 5.1.x and 6.x' it talks about a similar situation. So, with the Model 4 disk in drive 0 (48 TPI drive) and a blank 96 TPI disk in drive 1 (96 TPI drive), I followed the instructions:

FORMAT :1 (cyl=80,sides=2)

BACKUP SYS0/SYS:0 :1 (S)

BACKUP SYS/SYS:0 :1 (NEW)

However, I wasn't exactly sure what to do after the second backup instruction so I came up with:

BACKUP :0 :1 (SYS,INV,NEW)

This takes a long time! It may not be the correct way to do it but it indeed created a DOS boot disk that works in the 96 TPI drive. The disk has over 500KB free as opposed to 39KB on the 48 TPI disk, showing the big advantage of using higher capacity disks. Of course, once you have everything you want on the disk you should make another copy and write protect the original. I found that to successfully make a copy I had to run through the same steps as before, I couldn't just use BACKUP :0 :1.

Now that I had both Torch drives connected in the Model 4P and was able to boot off the newly created boot disk there was the issue of mounting the drives. Firstly, the power cable didn't reach since the power connectors are in a different place to the previous drives so I had to pull out the power cables a bit to get some extra length. Then there was the problem of actually screwing everything back together; while the drive mounting holes in the drive were in the same place as the drives that were previously installed in the Model 4P, I don't think the screws were quite in the right place when they were added to the metal that holds the drives. Additionally, you're supposed to cut the computer's front bezel to accommodate some drives, but I didn't fancy doing that. I ended up using different screws to hold the drives in place and had to leave one out and only just was I able to fit everything back in place.

At least now I had two working, high capacity drives that are matched, and from the outside the only difference is that the drives have the 'push-down door' type latch, whereas the Model 4P usually has the 'turn lever' type, which doesn't really bother me. Any software I need I can likely get online and either use my GoTek as an external drive to the Model 4P or I can create disks using my Greaseweazle.

PSU maintenance

As with many retro computers it's strongly advised to replace the RIFA safety capacitors in the power supply as they tend to crack over time and may explode while the computer is powered on, which usually doesn't do any harm to the machine but it's not nice having foul smelling fumes erupting out of the computer!

Do not work on any power supply or other high voltage circuit if you aren't familiar with such circuits and know how to protect yourself against danger.

Replace any RIFA branded capacitors - even if they look in good condition - with modern equivalents and replace any capacitors showing signs of going 'bad', such as leaking capacitors.

To get to the power supply PCB please see the Teardown section.

The following photo shows the PSU from my UK Model 4P, which differs from the US version, the video from Adrian's Digital Basement (see Repair/General) shows the US power supply.

This particular power supply is an Astec AZ 0180 which has the following ratings specified on the sticker:

Max. output power 50W (65W for forced air)

Max. circuit rating:

V1 +5V 5A V2 +12V 1.5A V3 +12V 2.1A V4 -12V 0.25A

The mains input power goes to the TB1 connector (top right in the image above) but there is also a connector near DB1 labelled 230V on one side and 115V on the other, which has unknown use.

To remove the PCB from the metal plate, take out the four screws in the corner of the circuit board; note that there is a plastic sheet between the PCB and the metal plate, make sure it stays in place. Remove the three leads (one on TB1, two on TB2) so that the PCB is completely free from the computer.

I was surprised to see that two of the capacitors had already been replaced (C1 and C13, highlighted in green in the above image) - at least I assumed they were at one point the RIFA type - so I only had to change the one RIFA capacitor, C2, outlined in red, which had signs of cracking upon closer inspection. As was common for Astec power supplies, it's possible to fit one of two different size capacitors (at least for C1 and C2 on my PSU version).

Resources

Introduction to your disk system TRS-80 Model 4P

https://www.manualslib.com/manual/1357964/Radio-Shack-Trs-80-Model-4p.html

Modem 4P Operation Manual

https://vintagecomputer.ca/files/Radio%20Shack/TRS-80%20Model%204%20Modem.pdf

Model 4P ROM Explained

https://www.trs-80.com/wordpress/roms/mod-4p-rom-disassembled/

Note: interactive, has instruction links to follow the code easily.

TRS-80 Model 4/4P technical reference manual

http://dunfield.classiccmp.org/trs80/m4tech.pdf

TRS 80 Model 4P Service Manual

https://archive.org/details/TRS80Model4PServiceManual/mode/2up

Software

The Model 4P doesn't have any programming language or O/S built-in, instead software has to be loaded from disk, which includes the Model III and Model 4 library. In particular, the Model 4P was sold with TRSDOS 6.1.1, but is also compatible with other versions of DOS, such as LDOS, as well as other types of O/S's, with CP/M Plus being an example. For convenience since it's difficult to capture video from a real Model 4P,  as we look at the various software I will provide screenshots from the TRS-80 emulator, TRS80GP, which you can read more about in the Emulation section.

Let's first briefly look at TRSDOS 6.1.1, a screenshot of which you can see below:

Notice the date toward the bottom of the screen; normally you would have to enter the date but the emulator automatically sets the date to the actual date of the computer the emulator's running on, though TRSDOS can only handle dates between 1980 and 1987, so it's chosen the latest year instead of the correct one. When the Model 4P is restarted, if a date was previously entered (either manually or through emulator magic) it will not prompt for the date anyway.

TRSDOS has a wide range of commands it recognises (which can be entered in lower or upper case), please see this site for a breakdown:

http://cpmarchives.classiccmp.org/trs80/mirrors/kjsl/www.kjsl.com/trs80/mod4trsdos.html

Let's start simple, there is the CLOCK command, which can return or set the time, but can also enable an always on clock display at the top left of the screen, using:

TIME (CLOCK=ON)

It can be seen the next image:

The clock will temporary vanish whenever the computer is busy, however.

The DIR command can be used to see the files on disk, the command on its own defaults to listing the visible files on all drives; drives are named as :0, :1, etc., as opposed to the more familiar A, B, and so on.

In the above screen capture I listed the files just on the first drive, using:

DIR :0

Microsoft BASIC comes with TRSDOS and it is run simply by using the BASIC command, when BASIC starts it will greet us with this screen:

Notice that the clock still runs, even in BASIC.

I wrote a simple program (see the listing below) which takes a start and end value from the user and outputs the values between and including the start and end values:

There is simple error checking to make sure valid start and end values are entered.

To save your BASIC program use the SAVE command, for example:

SAVE "TEST:1"

Will save the BASIC program as 'TEST' on drive 1.

To load the same program use:

LOAD "TEST:1"

You can issue DOS commands from within BASIC, for example, to do a directory listing of the first drive:

SYSTEM "DIR :0"

To exit back to DOS, use SYSTEM on its own. If you enter BASIC again your program will be gone so be sure to save it first. 

Back to DOS, we will look at another feature, this time setting up a virtual disk in memory, a MEMDISK, which is useful for storing commonly used files rather than having to keep loading them from disk, and for this it's helpful to have the RAM expansion.

First we use this command:

SYSTEM (DRIVE=5,DRIVER="MEMDISK")

You will get a screen like this:

Which will set up the memdisk as drive 5. The system will respond asking for the type of allocation (which RAM bank to use or disable the memdisk), single or double density, and whether is should be formatted. The memdisk will then be set up (provided there are no issues with the RAM), and you can then use the memdisk as if it were a 'normal' drive. The memdisk can't hold that much, however, even if you select to use Banks 1 and 2 (option D), results in a 63KB disk.

With the memdisk set up you can then copy over files to it, for example:

COPY DAY/TXT:1 TO :5

And then use the files directly from the memdisk as if it were a normal disk in a drive except that the memdisk is lost if the machine is reset or power cycled.

A handy testing/programming command is DEBUG, to use it first activate it:

DEBUG (ON)

Then press the break key (make sure there is no disk activity when you press the key) and you will enter debug mode which defaults to showing a register and memory dump:

See the previous link for how to use DEBUG but to give a sample of what it can be used for, you can move memory with the B command, display memory with D, fill memory with F, read/write from/to a hardware port using Q, search memory for a value using W, and exit back to DOS using O. Once back in DOS, use DEBUG (OFF) to disable debug mode.

Teardown

This section covers the teardown of my own Model 4P; please also see the Repair section for a video showing how to take apart the Model 4P (not my video).

First remove the two screws either side of the case toward the front and the two screws holding the handle in. You can then pull the large part of the case off, toward you (it may take a bit of an effort).

You'll find a large metal plate at the top, take out the six screws (three either side), and a further three screws at the rear near the plate. Now you can pull out the plate, which has the power supply PCB attached to it.

Looking inside the Model 4P we can see the video monitor CRT board taking up most of the space on the left, note the range of controls (H-WIDTH, INT-BRT, FOCUS, H-HOLD, V-SIZE, V-LIN, V-HOLD) which can be adjusted as needed but but should be left alone unless there really is need to alter them. The front panel controls are just visible under the PCB, the bottom of the CRT can be seen bottom right, and the power in mains socket and earth lead are visible top right.

Here is a view showing the CRT in the middle, the expansion power edge connector behind the CRT, and the system fan on the right:

In the next image we can see a closeup of the two disk drives and the supporting metal piece that holds them in place, which is attached to a larger metal piece. The two disk drives are daisy chained using power and data, there is no twist in the interface cable as the disk drives themselves set which drive is 0 and 1 via jumpers.

To remove the floppy drive enclosure which the two drives are secured to: take out the two large screws toward the edge of the computer. Take out the four smaller screws that attach the disk drive bracket to the other large metal piece. Remove the screw near where the single keyboard wire connects to the piece of metal. Take out the two screws between the disk drive bracket and the metal covering the motherboard; unscrew and use pliers to grab one screw at a time. Remove the power and data cables from both disk drives and lift up and out the disk drive unit. Note: the system fan is now exposed.

The disk drives can be removed by taking out four screws on each side (my 4P had only three on one side, possibly because one screw hole was drilled in the wrong place to fit the replacement drives). Drive 0 is set to drive 0 using a jumper on the disk drive and no resistor terminator pack fitted. Drive 1 is set to drive 1 using a jumper on the disk drive and has a terminator pack fitted.

The disk drives in my Model 4P have no branding but one has the model number 'NF 5300' (drive 0) and the other 'SM596D' (drive 1) and the drives appear to have been added at a later date perhaps as an upgrade (it was common to install double density drives) as additional screw holes have been added to accommodate the drives, and ‘0A’ and ‘1B’ stickers have been added to the front of the drives. Note that the data cable for the drive on the left is broken, the damage perhaps happening when the drives were installed, but the cable still works.

Additionally, you can just make out the large cooling fan behind the disk drives.

Technical

Memory

It's not possible to have the ROM and 64K RAM both enabled, see pages 117 to 119 of the TRS-80 Model 4/4P technical reference manual linked in the resources section for the memory maps.

Troubleshooting

Boot ROM version

To display the Boot ROM version, which could be useful for testing, hold the 'V' key and power on or press the reset button, which will result in a screen like this one:

You will not be able to do anything else until you power cycle or press the reset button.

RAM test

There is a built-in RAM test that you can run by holding either of the '.' (full stop/period) keys while powering on (or while pressing the reset button), press enter to start the test when prompted. The RAM test checks the RAM using various tests and any faulty RAM locations will be listed, this screen shows the Modified Address Test phase (the mask value that's displayed will change as the test is carried out:

Note that the built-in test only checks the first 64KB, if you have an additional 64KB you will need to use alternate software to do a complete RAM test.

The checks run constantly, so the only way to exit is to press the reset button or power cycle the computer.

If your machine isn't even booting it may be worth looking at a test ROM that can be installed into the machine such as the TRS-80 Diagnostic ROM found at:

https://github.com/misterblack1/trs80-diagnosticrom

The ROM can test the RAM of a Model I, II, III, and 4/4P.

If your system does have bad RAM then you will need to replace the faulty chips with 4164 (or equivalent) 200ns or better DRAMS, 5V only type.

80 column mode faulty

If 40 column mode works but 80 column mode is garbled/glitched, capacitor trimmer C231 may need adjusting or replacing, it's located on the motherboard. Please see Adrian's repair video series on the Model 4P linked in the Repair section for further help.

Upgrades

RAM

The Model 4P has sockets U153 to U160 for installing an additional 64KB RAM using x8 4164 DRAMs, 200ns, however, you must also move jumpers E12 and E13 to E11 and E12.

Emulation

It's always best to experience the original hardware but of course it's not always possible, especially due to the cost of the computer and peripherals, or even just availability on the second hand market. Emulators strive to recreate a retro computer on modern hardware but with many benefits, like ease of working with virtual disks, programming and debugging tools, to give a few examples, and even if you own the original hardware, likely an emulator will still be helpful, such as to aid with writing programs for the system. Since many of the TRS-80 computers feature a built-in CRT, and capturing screens with a camera doesn't result in the best images, emulators also have another advantage in that we can easily produce screen captures in clear form.

When I was first looking into emulators for the TRS-80 computer range I came across this site which had a very good comparison of a number of Tandy emulators:

https://www.trs-80.com/wordpress/emulators/

I decided to try TRS80GP which runs on Windows, Mac and Linux, and supports many TRS-80 computers including Model I,II, III, 4 and 4P, claiming to have accurate and near complete emulation. The emulator has built-in ROMs and DOS O/S's (TRSDOS & LDOS), and supports virtual floppy disks, cassettes, and hard drives, and allows capturing of the computer 'screen' and audio, as well as snapshots (the emulator's current state) but only for some emulated models. TRS80GP has advanced features including a built-in Z80 debugger, disk sector viewer, and disassembler.

The emulator can be downloaded from the creator's site:

http://48k.ca/trs80gp.html

Since I use Windows I was pleased to see that TRS80GP supports Microsoft's O/S from Windows XP to Windows 10. I downloaded Version 2.4.10, dated December 2022, showing the TRS80GP is still in active development.

After downloading the file and extracting it, to run the emulator I only had to double-click the executable, since I'm using Windows it's located in the windows folder, and is named trs80gp.exe. It's easy enough to use TRS80GP but be sure to read the home page for lots of information on using its features.

When you start the emulator for the first time it defaults to emulating a Model III with 48KB RAM:

It’s easy to change to a different machine by pointing to File->Change Model and selecting a different computer option:

The emulator remembers the last model that you selected (and other settings) for when you start the emulator again.

Now I've changed to Model 4:

You may have noticed that by default the emulator automatically boots into TRSDOS as that is selected by default for drive 0. It's easy to assign what’s in each of the disk drives using the Diskette menu, either selecting one of the presets or using the Insert disk… option. There are options to load a disk image and to export a virtual disk to file so it's easy to backup your virtual disks or even write to a real floppy drive with an appropriate floppy drive/controller and software.

Since DOS is running we have access to all the usual commands, with a very simple example shown in the next image of using the DIR command:

Only drive 0 has a disk image attached to it so you can see the listing for that drive, drive 1 is empty, and to see the contents of the other drives you simply press any key.

To take a screenshot, click Record->Screenshot (or F11) and the emulator will prompt you for a file name and location but only GIF format is supported. here is an example screenshot:

There is also a Cleanshot option (Shift+F11) but that is only concern for Model I as to produce a better looking image and avoid so-called beam drop-outs. The Record menu also has options for saving audio and video, the contents of RAM, and other features of the emulated computer.

Looking now at the advanced features, there is a disk viewer available from Debug->Disk Viewer…:

Not only is it a fascinating insight into how data is stored on disk it may also prove useful for repairing a bad disk that's been captured from a real drive.

Lastly, the Z-80 debugger, which is launced by selecting Debug->Z-80 debugger…:

Note that I paused the emulator (File->Pause/Resume or F9) when taking the screenshot as otherwise the disassembly listing continually changes.

The debugger is very helpful for learning how a TRS-80 works and for debugging your own machine code programs.

Peripherals and expansion hardware - various models

The various TRS-80 models enjoy a continued dedicated community, many of which produce new hardware for the Tandy computers, whether modern versions of original add-ons or original hardware that gives extra features to the computers. Please see the following links to read more about the many products developed for the TRS-80 computers.

TRS-80 Replacement Hardware – Add Ons

https://www.trs-80.com/wordpress/repairs/add-ons/

Resources - various models

Books

Machine language disk I/O & other mysteries

https://archive.org/details/Machine_Language_Disk_IO_and_Other_Mysteries_1983_Michael_J_Wagner

Microsoft BASIC decoded & other mysteries for the TRS-80

https://archive.org/details/microsoftbasicde0000farv

The custom TRS-80 & other mysteries

https://archive.org/details/Custom_TRS-80_and_Other_Mysteries_1982_Dennis_Bathory_Kitsz

TRS-80 interfacing

https://archive.org/details/TRS-80_Interfacing_Book_2_19xx_Jonathan_Titus

Magazines

80 Microcomputing (later shortened to 80 Micro) was an American TRS-80 oriented monthly magazine published by Wayne Green that ran from 1980 to 1988, containing type-in programs as well as software and hardware reviews. You can view archived copies at:

https://archive.org/details/80-microcomputing-magazine

(Note: incomplete as of February 2023.)

Technical - various models

RAM Addresses and Routines

https://www.trs-80.com/wordpress/ram-addresses-and-routines/

Ports and I/O Devices

https://www.trs-80.com/wordpress/ports-and-i-o-devices/

RS-232C

From the Model 4P service manual:

All Model I, III, and 4 software written for the RS-232-C interface is compatible with the Model 4P RS-232-C circuit, provided the software does not use the sense switches to configure the interface, The programmer can get around this problem by directly programming the BRG and UART for the desired configuration or by using the SETCOM command of the disk operating system to configure the interface.

Pinout description of the DB-25 connector (P1). 

1 PGND (Protective Ground) 

2 TD (Transmit Data) 

3 RD (Receive Data)

4 RTS (Request to Send) 

5 CTS (Clear To Send) 

6 DSR (Data Set Ready) 

7 SGND (Signal Ground) 

8 CD (Carrier Detect)

19 SRTS (Spare Request to Send) 

20 DTR (Data Terminal Ready) 

22 RI (Ring Indicate)

To add to what the service manual says and to give context, the Model I used an add-on board to provide an RS-232 interface and it was the add-on board that had switches to do the configuration.

In the pinout above, the pin numbers not listed have no connection (at least on the Model 4P, which is what I tested). Be aware that the signals use +/- 12V so make sure any circuit or device that you interface with the Model 4P's RS-232C port is compatible with those voltage levels or makes the necessary voltage level conversions.

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